Why do we love?

What makes us fall for that person. (Or in some cases those PEOPLE..)

why is it simple things about them like their smell, their smile or even the way the walk can send our heart into flutters?
Why is it that it sneaks up on you and hits you with the force of a train going at 120mph?

We don’t choose to love, certainly not. Just another one of those things affected by those around us.

Why is it a lot of people fall in love when the people we meet are so random, so really you could fall in love with anyone?
Anyone know what is it that makes us chase after someone?
Is it a chemical thing, or physical or a mix?

All these feelings and I stop to wonder, why?
If anyone can shed any light on it, I know this question isn’t very clear at all, but just any ideas of what makes us fall for someone.
is it that we are attracted to people that remind us of our parents (those who nurtured us?)
or those of a similar race
or with a certain temperament, smell, voice, size

ANYTHING at all
if you know anything i would be extremely interested.
also why we have no control over it, why we continue to love even when it is unrequited and puts us in such pain?